CDI Interpretation. With your receiver properly tuned and checked,
rotate the OBS knob until the needle centers. Assume that the needle centers
with the 180° course under the index (Fig. 8-2). Your aircraft can
be on any heading and at any point over the north/south reference line
shown, except over the station or close to it. Approaching inbound from
point "I" to point "S", the CDI will deviate from side to side as the aircraft
passes directly over the station where no signal is received. Likewise
a centered CDI with any other course shown under the index locates your
aircraft over either the selected radial or its reciprocal.

Figure 8-2. CDI interpretation.

To/From Interpretation. With a course of 180° selected under
the index, a "TO" indication shows, regardless of heading, whenever your
aircraft is located within the approximate hemispherical area north of
the 90/270 reference line. A "FROM" indication appears when the aircraft
is located south of the approximate 90/270 reference line. Movement across
the 90/270 area is indicated by an ambiguous TO/FROM signal. In this area,
the resultant of the opposing reference and variable signals, which actuate
the TO/FROM indicator, is insufficient to produce a positive "TO" or "FROM"
indication. At a speed of 150 knots, the approximate times to cross this
area at various distances from the station (Fig. 8-3) are as follows:

Likewise, with any given course under the index, a "TO" will show
whenever your aircraft is located in the hemispherical area approximately
90 degrees on either side of the selected course. "FROM" will show in the
other hemisphere. Note that the course index is at the bottom of the dial
shown in Figure 8-4.

CDI and To/From Interpretation. As Figure 8-4 shows the position
of the CDI tells you that: You are on a specific radial (or its reciprocal);
or you are to the right or left of it; with respect to the course selected.

Figure 8-3. To/From interpretation.

Movement of the CDI, in conjunction with heading information,
tells you:

Drift correction and time/distance checks will be discussed later
in this chapter.

The TO/FROM indicator locates you in one of two hemispherical
areas, depending on the course selected, or in an area of weak signal strength.

To determine the radial on which you are located:

1. Note the TO/FROM reading.
2. If the reading is "FROM", center the CDI by rotation of the
OBS knob opposite the direction of CDI deflection. When the needle centers,
read the radial under the course selector index.
3. If the reading is "TO", rotate the OBS knob in the direction
of CDI deflection until "FROM" appears and proceed as in the previous step.

To determine the inbound course to the station from your present
position:

1. Read the inbound magnetic course directly from the reciprocal
window, following determination of the radial (if your indicator has no
reciprocal window, compute the inbound course arithmetically).
2. Rotate the reciprocal to the opposite course index to change
the "FROM" to a "TO" reading, and the needle will center, provided your
aircraft has not moved to another radial. If so, recenter the needle with
small adjustments, unless you are close to the station and crossing radials
rapidly.

Figure 8-4. VOR orientation.

3. To "home" to the station, turn to the heading indicated by
the course index. Continue to recenter the needle and adjust the heading
as necessary. Since homing does not involve drift correction, the aircraft
follows a curved path to the station in a crosswind condition.